Telephone system



0ct .14,1941. s. l-zlfa mnsan mm V 2,258,660,

TELEFHONE SYSTEM Filed April 11, 1940 1o Sheets-Sheet 'r //v TERRUP m?75 TRANS RELAYS a0 m/ws CONT/z 85 RING/N6 com/Emma FOR GENERATINGFREQUENCIES FL 5 F2 RYIO R720 732 MORRIS E. GRIFFINS INVENTORS ,F' 7 sewE. PETERSON ATTORN EYS Oct. 14,

' lNTERRUP TER 75 S. E. PETERSON El At TELEPHONE SYSTEM 118d April 11,1940 10 Sheets-Sheet 9 i MASTER 77MI/VGARELAYS 95 v 9Z4 I r6870 E Iv 1 vv R910 W p -9 4/ H 2900 I 92/ R950 902 r6873 92 5? [C874 wag H5930 I1:1?940 x 6767 \F Qrv INVENTORS SETH E. PETERSON MORRIS E. GRIFFINS BY 9E:

ATTORN EYS Patented Oct. 14, 1941 2.258.660 TELEPHONE SYSTEM Seth E.Peterson, Chicago, and Morris E. Griifins, Riverside, Ill., asslgnors toAssociated Electric Laboratories, Inc., Chicago, 111., a corporation ofDelaware Application April 11, 1940, Serial No. 329,087

53 Claims. (Cl. 179-27) The present invention relates to telephonesystems, and more particularly to improvements in apparatus forproviding call intercepting and supervisory services in conjunction withthe use of the lines and automatic switching apparatus nected from theline, it is desirable to provide an arrangement whereby calls intendedfor the disconnected substation are routed to an operators position sothat the calling subscribers may be informed that the called substationis no longer in service. Call intercepting service of this character isalso desirable in other in-' stances when calls to substations actuallyin service are to be routed to an operators position.

It is an object of the present invention to provide improved apparatusof the character described, which is of simple and economicalarrangement and which functions in a positive and reliable manner tointercept calls to one or more predetermined substations or lines of thesystem and automatically to route the intercepted calls to an operatorsposition.

It is another object of the invention to provide improved apparatus ofthe character described, which is so arranged that the control of thecall intercepting apparatus is effected through operation of a ringinginterrupter which is common to all of the lines of the system.

It is a further object of the invention to provide improved apparatus ofthe character descrlbed,-which is so arranged that the operation thereofto intercept a call intended for a predetermined line or substation mustbe completed within a predetermined time interval after operation of theapparatus is initiated, and if not completed within this time intervalthe apparatus is automatically released.

It is still another object of the invention to provide improved callintercepting apparatus of the character described, which is arranged tointercept calls intended for substation designated by ringing currentsof different codes and diiferent frequencies and wherein provisions aremade for grouping the substations so thatthe substations arranged forcall intercepting service and designated by ringing currents of the samefrequency are served by apparatus controlled only by ringing currents ofthis frequency.

It is a. further object of the invention to proconnections between thelines.

vide an improved ringing interrupter which is of simple and economicalarrangement and which operates in a positive and reliable manner toprovide differently coded ringing voltages of different frequencies foruse in the selective signalirig of called substations associated withthe multiparty lines of a telephone system.

It is another object of the invention to provide an improved interrupterswitch of the character described, which includes provisons fordeveloping call intercepting impulses individual to the various codedringing voltages of difierent frequencies in a predetermined timedrelationship with these voltages, and which further includes apparatusfor developing check pulses in timed relationship with the interceptingpulses for the purpose of controlling an associated call interceptingcircuit to prevent the false operation and lockup of the circuit.

It is still another object of the invention to provide in an automatictelephone system including directively controlled switches for settingup connections between the lines of the system and apparatus forproviding supervision of the switching equipment, an improvedarrangement whereby on supervisory test calls the backbridge relays ofthe connector switches in use are prevented from operating.

The invention is illustrated in its embodiment in an automatic telephonesystem which comprises a plurality of subscribers lines, several ofwhich are of the multiparty type, together with automatic switchingequipment for setting up The system further comprises improved ringingapparatus of the coded ringing type for selectively signaling thesubstations associated with the multiparty lines of the system. Moreparticularly, this apparatus includes two sources of ringing current ofdifferent frequencies and a ringing interrupter switch which functionsto interrupt the voltages of the two sources to provide differentlycoded ringing voltages of the two frequencies. This .ringing interrupterincludes a plurality of ringing leads, a plurality of call interceptingleads, a check pulse lead, and cyclically operative coding relays forimpressing the differently coded ringing voltages upon the ringing leadsduring each cycle of operation thereof, and for impress-- me apredetermined potential upon the intercepting leads during predeterminedintervals which individually-correspond to the different codes. Theringing interrupter iurther includes a circuit arrangement whereby checkpulses are impressed upon the check pulse lead during the cycles ofoperation thereof, ringing voltage of 5 one frequency is coded andapplied to the corresponding ringing leads, and during the interveningcycles of operation thereof ringing voltage of the other frequency iscoded and applied to the remaining ringing leads.

The arrangement of .the system is such that the ringing voltages,intercepting pulses and check pulses developed through operation of theringing interrupter, are utilized to control a pair of call interceptingcircuits which respectively respond to ringing voltages of the twodifferent frequencies to intercept predetermined calls routed to certainof the lines or substations of the system. The operation of each ofthese two circuits is under the joint control of'the ringing voltagesapplied to the associated lines arranged for call intercepting serviceand the application of the intercepting pulses to the interceptingleads, the arrangement being such that the application of anintercepting pulse to a predetermined one of the leads must partiallyoverlap the application of ringing voltage to one of the lines arrangedfor intercepting service before operation of the intercepting circuit isinitiated. After operation of an intercepting circuit is initiated, thecircuit must complete its operation to intercept the call before a checkpulse is impressed upon the check pulse lead. Otherwise, certain of therelays embodied in the intercepting circuit function to release thecircuit. By virtue of this arrangement, the intercepting circuits areprevented from being inadvertently locked up due to faulty relayoperation or transient voltages impressed upon the lines arranged forintercepting service.

Further features of the invention relate to the particular arrangementof the circuit elements whereby the above and additional operatingfeatures of the system are attained.

, The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. Theinvention, bothas to its organization and method of operation, together with furtherobjects and advantages thereof, will best be understood by reference tothe specification taken in connection with the accompanying drawings inwhich Figs. 1 to .9, inclusive, taken together, illustrate an automatictelephone system having embodied therein the features of the inventionbriefly outlined above; Fig. graphically illustrates the mode ofoperation of the ringing-interrupter forming a part of the systemillustrated; and Fig. 11 illustrates the manner of combining Figs. 1 to9, inclusive, to obtain a unified system.

Referring to the drawings, the system there illustrated comprises anexchange at which are terminated a plurality of single party subscriberslines, one of which is indicated at it as extending to the substation A,and a plurality of multiparty subscribers lines, one of which isindicated at it as serving the substations B, C and D. For the purposeof setting up connections between the lines of the system automaticswitching equipment is provided which includes line switches, aplurality of line switch-es, two of which are indicated at [5 andrespectively, a plurality of selector switches including the switch 25,and a plurality of connector switches including the switch 30. Thesystem further comprises a super.

visory operator's position 2! which may be provided in the exchangeserving the above described single party and multiparty subscriberslines or in a distant exchange, as desired. This operators positionterminates one end 'of a trunk which extends to an incoming selectorswitch 22 having access to the various connector switches of the system.The line switches I5 and 2G and the other similar switches'of the systemare nonnumerical switches in the sense that they are inefiective toperform any line selecting function other than those of selecting idleones of the selector switches. The selector and connector switcheson-the other hand, are commonly known as numerical switches in that theyare directive- 'ly controllable from the telephone stations of thesystem to perform particular line selecting functions. It will beunderstood that a plurality of switches of each type are provided in thesystem for performing the various line switching operations. Preferably,eachof the line switches 85, 20, etc., is of the well known rotary typeof which there are several commercial embodiments. Each of the selectorand connector switches is preferably of the well known Strowger typehaving embodied therein a wiper carriage structure adapted to betranslated vertically to bring the wipers thereof to a position oppositeto a particular level of bank contacts, and then rotated to bring thewipers into engagement with a particular set of contacts in a selectedlevel; and control relays suitably connected and arranged to control theenergization of the various magnets and the line switching in accordancewith the particular function assigned to the switch. 0bviously, theparticular arrangement of the control apparatus embodied in the switchesof different type, is different in each instance, depending upon thecharacter of the switch.

In order more clearly to explain the operation of certain of thecircuits to which the present invention pertains, a portion of the relayequipment embodied in the connector switch 36 has been shown in detail.More particularly, this equipment comprises a slow-to-operate ringingcontrol relay RI Ill, a slow-acting ring cut-pfi relay Rlllfi, aback-bridge or battery reversing relay R90, and a ringing code selectingswitch I23. The switch I26 includes a wiper m arranged to traverse a setof associated contacts terminating ringing leads having differentlycoded ringing voltages impressed thereon.

In order to develop the difi'erently coded ringing voltages which areutilized in selectively signaling the subscribers served by'themultiparty subscribers lines, there is provided ringing apparatus whichincludes two sets of ringing converters diagrammatically illustrated atand an improved ringing interrupter 15. Each set of ringing generatorscomprises a pair of converters, preferably of the well known thump-startvibrating type, for developing ringing voltages having the frequenciesF|- and F--2, respectively. In general, the arrangement of the ringinginterrupter 15 is such that three differently coded. ringing voltages ofeach frequency F-i and F-Z are provided Briefly described, thisinterrupter comprises a group of ringing leads, ClEii to C1563,inclusive, which are labeled according to the frequency and code of theringing voltages applied thereto; a check pulse lead; (2689; a group ofcall intercepting leads C151 to Cit-i2, inclusive, which are alsolabeled according supervisory leads including the leads C606, C140, C110and C100; ,a pick-up pulse lead CH6: and a plurality of control relaysfor selectively controlling the application of diiferent voltages andpotentials to the various enumerated leads. More specifically, the relayequipment of the interrupter 15 comprises a group of coding relays 60, agroup of master timing relays 35, and a start relay R100. The codingrelays are, when the start relay R100 is energized, operative in acyclic manner under the control of the cyclic operation of the mastertiming relays 05, and function during alternate cycles of operationthereof, to

impress ringing voltage of the frequency F--l upon the three leads Cl,C153 and C155. At the end of these alternate cycles of operation of thecoding relays 90, the ringing circuits are rearranged through operationof a set. of transfer relays 80 and a set of transfer control relays 85,so that during the intervening cycles of operation of the coding relaysringing voltage of the frequency F-2 is impressed upon the ringing leadsC152, C154 and C156. The transfer and transfer control relays alsofunction to control the intercepting pulse circuits so that interceptingpulses are impressed upon the 'F-l intercepting leads C151, C156 andC153 during alternate cycles of operation of the coding relays, and uponthe F--2 intercepting leads C160, C15! and C162 during the interveningcycles of operation of these relays.

For the purpose of intercepting calls intended for predetermined linesor substations of the system, there are provided two call interceptingcircuits, and 36, which are of identical arrangement and are associatedwith the lines arranged for intercepting service. These circuitsfunction to route intercepted calls to an intercepting operatorsposition over the trunks 33 and 31,

respectively. Briefly described, the call inter-,

cepting circuit 35 comprises a plurality of line connect relays, ofwhich one relay, R2 I 0, is shown, which are individually associatedwith the lines arranged for call intercepting service. The circuitfurther comprises a plurality of code relays, R220, R230 and R240 whichare common to the associated linesand in conjunction with thealternating current ring-up relays,.o'f which one relay, R200, is shown,function selectively to control the energization of the line connectrelays. The common relay equipment of the call intercepting circuit 35also includes a secondary ring up relay R320, a slow-acting ringcut-oil! relay R350, a back-bridge relay R330, a pair of control relaysR300 and R310, and aslow-to-operate check pulse controlled relay R340.In the arrangement illustrated, the code relays R220, R230 and R240, areindividually controlled over they leads C159, C158 and C151 byintercepting pulses corresponding to the coded ringing voltages havingthe frequency F-l and the codes I,

2 and 3, respectively. Accordingly, the circuit 35 is only utilized inintercepting calls to substations designated by ringing voltages havingthe frequency F-l. The call intercepting circuit 36, on the other hand,is arranged to intercept calls to predetermined substations designatedby codedquency in this manner, a more simplified arrangement' isobtained which is considerably more reliable in operation.

The trunk circuit 4| terminating the trunk 31 is identical with theapparatus provided at the operator's position 40 to terminate the trunk38 and is of substantially conventional arrangement.

Briefly described, this apparatus comprises an answer key 361 forconnecting the operator's telephone set 310 to the trunk 39, a hold key363, an incoming call signal lamp 366, and a ring-up relay R360 which isbridged across the conductors of the trunk 30 to respond to ringingcurrent transmitted over this trunk and is arranged to control theenergization of the signal lamp 366. An impedance element 362 is alsoprovided which functions as a holding bridge across the conductors ofthe trunk 39 under the control of the hold key 363.

The common. equipment provided in conjunction with the automaticswitching equipment described above, also includes test and supervisoryapparatus for indicating the condition of the power and switchingequipment and for enabling a spervisory, operator, such, for example, asan,

operator attending the position 2|, to obtain an indication of thecondition of this equipment. In general, this supervisory and testapparatus is arranged to provide two classes of signals indicative offaults present in the power and switching equipment of the exchange.Thesesignalshave arbitrarily been designated as no-danger alarm signalsand emergency alarm signals, a signal of the first type indicating thata fault is present in certain of the apparatus of the exchange whichshould be corrected but does not require immediate attention, andsignals of the second type indicating that a fault is present in theapparatus of the exchange which is extremely serious and a hazard tocontinued operation of the system. The no-danger alarm signals are underthe control of a group of relays 45 which include a nodanger fuse alarmrelay R400 and three control relays, R410, R420 and R430. Similarly, theemergency alarm signals are controlled through the provision of a secondgroup of relays 50, which'includes an emergency fuse alarm relay R440and three control relays R450, R460 and R410. Access to the variousalarm circuits controlled by the relays just described is obtainedthrough the automatic switching equipment of the exchange, a group ofsupervisory test relays 60 being accessible to the connector switches ofthe system over the supervisory trunk l3. This group of supervisory testrelays comprises a coding or pulsing relay R550, a pendulum type startrelay R560, and a slow-to-operate relay R510 which is provided for thepurpose of controlling the pulsing relay R550.

In order to control the condition of the various alarm circuits inaccordance with the condition of the ringing apparatus briefly describedabove, there is provided a group of ringing apparatus test relays 65,which operate through a test cycle each time operation of the associatedringing apparatus is started, and function to set the emergency alarmrelays 50 to indicate an emergency alarm condition in any case when theassociated ringing equipment becomes faulty. Briefly described, thisgroup of relays comprises an alternating current start relay R640, apair of check pulse controlled relays R620 and R630, a timer controlrelay R600, and an alarm circuit control relay R6l0. In this regard itis pointed out that the alarm relays forming the two groups 40 and 50are arranged to be controlled by a timer 55 which is of the all-relaytype and comprises five relays R500, R510, R520, R530 and R540.

As indicated previously, the power and supervisory equipment includesduplicate sets of ringing generators and supervisory tone sources.Thus,,the ringing converter equipment 185 was described as including twosets of converters for developing ringing voltages having thefrequencies F-l and F-2, respectively. Similarly, duplicate sets of dialand busy tone generators, respectively indicated at 000 and 005, areprovided for developing the dial and busy tone currents. Provisions aremade in the common equipment whereby the two sets of tone currentsources and the two sets of ringing generators may selectively beconditioned for operation from the supervisory operator's position 21,for example, by the simple expedient of dialing different test numbersrespectively designating the two sets of equipment; To this end there isprovided a group of transfer relays 10, which comprises a transfer relayR680 and a pair of transfer control relays R010 and R000, the last ofwhich is the slow-to-operate type. These relays not only enable asupervisory operator to condition either set of equipment for operation,but in addition rearrange the control circuits for the ringing'apparatustest relays 05 so that these relays will operate to perform theirassigned test functions regardless of the set of ringing and toneequipment which is conditioned for use.

Operation of the ringing interrupter 75 In order to facilitate theexplanation of the operation of the system as a whole the operation ofthe ringing interrupter 15 will first be considered. Operation of thisapparatus is initiated in response to the appalication of groundpotential to the start conductor C110, thereby to energize the startrelay R100. It will be understood that this start lead is connected togroundin response to seizure of any one-of the final nu.- mericalswitches of the'system. When this lead is connected to ground a circuitextending by way of the choke coil 133 is completed for energizing thestart relay R100. This relay in operating connects the F-1 ringingleads, C151, C153 and C155'to the ringing conductors C112, C110 andC114, respectively; and the F-2 ringing leads C152, C154 and C150 to thecommon ring cut-oil conductor C111. Thus, the F-1 ringing lead C151 isconnected through the contacts 103 and 111' to the ringing conductorC112 when the start relay R100 operates. Similarly, the second F-!ringing lead C150 is connected through the contacts 105 and 110 to thesecond ringing conductor C113; and the third F-1 ringing lead C155 isconnected through the contacts 101 and 115' to the third ringingconductor C110. The F-2 ring leads, C152, C100 and C150 are connected tothe ring cut-off conductor C111 over obvious paths. This ring cut-oficonductor may be connected directly to ground or to the negative ground.On the other hand, if the final selector switches of the system arearranged for battery imposed ringing, the X wiring shown in Fig. 1

of the drawings will be used whereby the ringing cut-on conductor C111is connected to ground through the exchange battery.

The relay R100, upon operating also completes at its contact 102, acircuit for energizing the start relay R000, thereby to initiate a cycleof operation of the ringing apparatus test relay group 05 in the mannersubsequently explained. At its contacts 100, the relay R100 prepares acircuit for transmitting pick-up pulses over the pick-up lead C115 tothe ringing relays conventionally embodied in the final selectorswitches of the system. At its contact 101, the relay R100 completes anobvious path for grounding the release conductor C101, thereby toenergize the relays R000 and R010. The circuit for energizing the relayR000 extends from ground by way of the contacts 101, C101, the windingof R800, the resistor 806, the contacts 822 and 803, C165, the contacts102 and the resistor 108, to battery. The circuit for energizing theslow-to-operate relay R910 extends by way of the grounded conductorC161, the winding of R910 and the resistor 920, to battery. The relayR910 in operating opens at its contacts 911 the path normallyshort-circuiting the.slow-to-operate relay R920. At its contacts 912,the relay R910 prepares an operating circuit for the relays R810andR820.

The relay R000 in operating completes at its I contacts 805 a holdingcircuit for itself, this holding circuit extending by way of thegrounded conductor C101, the winding of R000, the contacts 005, C108,the contacts 812, 033 and 852 in parallel, C109, and the resistor 148170battery.- At its contacts 001 the relay R800 completes one of themultiple paths for short-circuiting the relay R020 thereby to preventthe latter relay from operating in response to operation of the relayR010. More specifically, this path extends short-circuiting the relayR130. At its contacts 803 the relay R000 completes a circuit forenergizing the two relays R850 and R800 in series, this circuitextending by way of the grounded conductor C161, the winding of R050,the contacts 803 and the winding of R860 to battery.

At its contacts 000 the relay R000 completes a circuit for energizingthe two relays R030 and R000 in series, this circuit extending by way ofthe grounded conductor C101, the winding of R830, the contacts 804, andthe winding of R000 to battery. Thus, the relays R030, R800, R050 andR060 are all caused to operate in response to operation of the startrelay R100. The relay R030 in operating, completes at its contacts 801,an obvious holding circuit for itself and the relay R000. At itscontacts 802, the relay R000 completes the prepared series operatingcircuit for the relays R010 and R820, this circuit ex= tending by way ofthe grounded conductor C161, the winding of R810, C010, the contacts012, C013, the contacts 032. and the winding of R020 to battery. At itscontacts 833, the relay R030 opens one of the above-traced parallelholding circuits for the relay R800. The relay R800 in operating,completes at its contacts 851, an obvious holding.circuit for itself andthe relay R000. At its contacts 052, the relay R050 opens a second ofthe multiple holding circuits for the relay R800.

The ringing relay R800, upon operating, opens at its contacts 801 apoint in the common portion of the paths, traced hereinafter, forshort-cir cuitingwhe relays" R010 and R000. At its contacts 002,- therelay R000 prepares a path for short-circuiting its associated controlrelay R050. At the remaining contacts controlled by the ringing'relayR050, certain additional circuit control operations are performed, whichare described with particularity hereinafter. The ringing relay R040,upon operating, opens at its contacts 04!, a-.further point in the pathfor short-circuiting the relay Ri0 and prepares, at its contacts 042,

the path for short-circuiting the control relay R000. At its contacts040, the relay R040 opens a point in the above traced operating circuitfor the relay R000. At the'remaining contacts controlled by the relayR040, additional circuit control operations are performed which aredescribed in detail hereinafter.

The relay R020, upon operating, further prepares, at its contacts 02!,the above-mentioned path for short-circuiting the relay R0l0 and opens,at its contacts 022, a further point in the operating circuit for therelay R000. At its contacts 020, the relay R020 prepares a circuit forenergizing the relay R100. At its remaining con tacts the relay R020performs additional circuit control operations described hereinafter.

Shortly following the operation of the relay R020, the slow-to-operaterelay R0l0 operates to complete a locking circuit for itself and therelay R020, this locking circuit extending by way of, the groundedconductor 0151, the winding of R0! 0, the contacts 0! and the winding ofR020 to battery. At its contacts 0!2, the relay ROI0 interrupts theholding circuit for the relay R000, causing the latter relay to release.At its contacts 005, the' relay R000 in releasing, opens a further pointin its holding circuit. At its contacts 000 and 004, respectively, therelay'R000 opens the above-traced operating circuits for the relaysR000, R040, R050 and R000. At its contacts 002 the relay R000 repreparesone of the operating circuits for the relay R040. At its contacts 00lthe relay R000 interrupts the abovetraced path short-circuiting thewinding of the relay R020. 7

The slow-to-operate relay R020 now operates to complete at its contacts02!, an obvious circuit for energizing the slow-to-release relay R000.The relay R000, in operating, completes at its contacts 00!, an obviouspath for short-circuiting the winding of the relay R0l0, causing thelatter relay to restore. At its contacts 002, the relay R030 completes acircuit for energizing the relay R000, this circuit extending by way ofthe grounded conductor 0101, the contacts 002, 042 and 052, and thewinding of R000 to battery.

The operation of the relay R000 marks the end tacts 022, 042, 052 and002, the winding of R050, and the contacts 04! back to the groundedconductor C101. At its remaining contacts the relay R000 performscertainadditional circuit control operations described hereinafter.

The relay R0|0 is deenergized and restores when its winding isshort-circuited'by the relay R000. Upon restoring, the relay R0!0recompletes the path for short-circuiting the winding of the relay R020,and at its contacts 0l2 opens the previously traced operating circuitfor the relays R0l0 and R020. The relay R020 restores shortly followingthe release of the relay R0l0 and upon restoring, opens the operatingcircuit for the relay R000. The relay R000 in turn restores after aninterval to interrupt, at its contacts 002, the above-traced path,short-circuiting the winding of the relay R050. As-a result, the tworelays R000 and R050 are energized in series over a circuit extending byway of the grounded conductor C101, the contacts 04!, the winding ofR050, the contacts 002 and the winding of R000 to battery. Thecompletion of this circuit prevents the release of the relay R000 andresults in operation of the relay R050. The relay R050 in operating,opens at its contacts 052, a further point in the operating circuit forthe relay R000 and prepares a circuit for energizing the relay R040 atits contacts 05!.

The relay R000 in releasing, also opens the path short-circuiting thewinding of the relay R0l0, whereby the latter relay is caused to re--operate and open the path short-circuiting the winding of the relayR020. In reo'perating, the relay R0! 0 also completes the initialltraced circuit for energizing the relays R020 and R0l0 in series.-Shortly following operation of the relay of the first ringing period.This relay in operating, completes at its contacts 00!, a holdingcircuit for the relay R000 and'in so doing short-circuits the relayR050. This holding circuit extends by way of the grounded conductorC101, the contacts 00!, 0012, the contacts 052 'and the winding of R000to battery. Obviously, with this circuit completed, both windingterminals of the relay R050 are groundedso that the relay R050 isshort-circuited and restores. The relay R050 in restoring, opens theinitially completed holding circuit for itself and the relay R000, andreprepares at its contacts 052, the holding circuit for the relay R000.The relay R000 in operating, also completes a path for short-circuitingthe winding of the relay R050, this path extending by way of thegrounded conductor C101, the con- R0l0, the relay R020 reoperates torecomplete the operating circuit for the relay R000. The last-mentionedrelay in operating, again shortcircuits the winding of the relay R0!!!and com pletes the above-mentioned circuiting for energizing the relayR040. This circuit extends from ground by way of contacts 10!, 0101, thecontacts 002, 042 and 05! and the winding of R040 to battery. Whenenergized over this circuit the relay R040 operates to complete at the.X contacts 040, an obvious locking circuit for itself. After thislocking circuit is completed the relay R040 opens its contacts 042 tointerrupt its own operating circuit and the operating circuit for therelay R000. Atits contacts R0, the relay R040 interrupts theabove-tracedseries holding circuit for the relay R000 and R050, causingboth of these relays to restore. The relay R050 in restoring, opens atits contacts 05!, a further point in the operating circuit for the relayR040, and reprepares at its contacts 052, the previously tracedoperating circuit for the relay R000. The relay R000 in releasing, opensthe above-traced holding circuit for the ringing relay R000. The ringingrelay R000 now restores to open at its contacts 002 a further point inits own holding circuit, and further to prepare, at its contacts 05!,the holding circuit for the relay R040 and the path for short-circuitingthe winding of the relay-R000.

The relay R0l0 in releasing, initiates the se-. quential release of thetwo relays R020 and R000 in the manner explained above. When the relayR000 restores, it again opens the path shortcircuiting the winding ofthe relay R0l0, and opens, at its contacts 002, the holding circuit forthe relay R040. The relay R040 now restores to reprepare the seriesholding circuit for the two relays R900 and R990 and the operatingcircuit for the relay R900. From this point on the master timing relays95 continue to interact, in the exact manner explained above, it beingpointed out that the release of the relay R900 marks the end of thesecond ringing period. Thus, each operation of the relay R900 marks theend of an odd-numbered ringing period and each release of this relaymarks the end of an even-numbered ringing period. At the beginning ofthe :fourth ringing period when the relay R900 reoperates, theabove-mentioned holding circuit for the relay R940 is completed, thiscircuit extending by way of the grounded conductor C191, the contacts90!, C912, the contacts 99! and 942, and the winding of R940 to battery.It will be observed that when this circuit is completed the upper andlower winding terminals of the relay R990 are both connected to ground,thus effectively short-circuiting this relay. As a result, the relayR990 is deenergized and restores to open at its contacts 93! theinitially completed holding circuit for itself and the relay R940. Atits contacts 932 the relay R930 opens the previously traced circuit forenergizing the relays R9l0 and R920 in series. At its contacts 933, therelay R990 prepares a second of the three multiple branches of theholding circuit for the relay R800.

At the end of the fourth ringing period, the relay R900 is deenergizedand restores in the manner explained above. This relay in releasing,opens at its contacts 90! the holding circuit for the relay R940. Therelay R940 now restores to open, at its contacts 942, a further point inits holding circuit as traced above. At its contacts 94!, the relay R940prepares a further point in one of the holding circuits for the relayR920. At its contacts 943, the relay 940 further prepares the previouslytraced operating circuit for the relay R900. At its contacts 943 therelay R940 also completes a circuit for energizing the relay R130. thiscircuit extending from ground by way of the contacts 10!, the winding ofR130, the contacts 143. C199, the contacts 923 and 943, C195, thecontacts 142 and the resistor 149 to battery.

At the end of the sixth ringing period as measured by the sixth cycle ofoperation of the mas- At its contacts 132, the relay R190, upon. oper- Iating, prepares another multiple branch in the holding circuit for therelay R900. At its contacts 13!, the relay R190 completes a holdingcircuit for itself, this holding circuit extending from ground by way orthe contacts 10!, the winding of R130, the contacts 13!, C199, thecontacts 902, C194, the contacts 145 and the resistor 149 to battery. Itwill be noted that the relay R140 is at this time shunted by theresistor 149. Accordingly, this relay does not operate at this time.

At the end of the fifth ringing period as measured by the fifth cycle ofoperation of the master timing relays 95, the relay R900 is againenergized and operates to complete the abovementioned holding circuitfor the relay R920, this circuit extending by way of the groundedconductor C191, thecontacts 90!, the conductor C912, the contacts 99!,84! and 92!, and the winding of R920 to battery. It will be observedthat when this circuit is completed, both winding terminals of the relayR9l0 are connected to ground and accordingly this relay is deepergizedand restores. In releasing, the relay R9! 0 reprepares, at its contacts9l2. another multiple branch in the holding circuit for the relay R900.At its contacts 9! the relay R9!!! opens a fil ter timing relays 95, therelay R990 is again deenergized and restores to openthe above-tracedholding circuit" for the relay R920. When this circuit is opened, therelay R920 is deenergized and restores to open a further point initsholding circuit. At its contacts 822, the relay R920 completes thepreviously traced operating circuit for the relay R900. At its contacts823, the

relay R920 opens the previously traced operat-.

ing circuit for the relay R130.

The relay R900, in reoperating at the end of the sixth ringing period,again completes, at its contacts 90!, the previously traced pathforshort-circuiting the winding of the relay R920.

At its contacts 803 and 904, the relay R900.

recompletes the circuits for energizing the relays R890, R940, R950 andR990, causing all of these relaysto reoperate. At its contacts 905, therelay R900 recompletes the previously traced holding circuit for itself.

resistor 149 is connected in shunt with the winding of the relay R140,thus permitting the relay R140 to be energized in series with the relayR190 over a circuit which extends from ground by way of the contacts10!, the winding of R190, the contacts 13! and the winding of R140 tobattery. The current flow over this circuit is suflicient to hold therelay R130 operated, and to cause the operation of the relay R140. Inoperating. the relay R140 opens, at its contacts 149, one of themultiple holding circuits for therelay R900. At its contacts 14!, therelay R140 prepares a path for connecting the resistor 909 in shunt withthe winding of the relay R130. At its contacts 142 and 143, the relayR140 opens a further point in the operating circuit for the relay R130.At its contacts 145, the relay R140 opens a further point in the pathincluding the resistor 149 which normally shunts its own winding. At itscontacts 144 the relay R140 further prepares the path forshort-circuiting the winding of the relay R130. At its contacts 141, therelay R140 completes an obvious circuit for energizing the two transferrelays R1l0 and R120 in series.

sixth ringing period. At its contacts 122, the relay R120 in operating,disconnects the common ringing current conductor C110 from the F-!ringing converter output lead C150. At its contacts 12!, the relay R120connects the common ringing current conductor C110 to the F2 ringingcurrent lead C150 extending to the ringing converter having the outputfrequency F2. Thus, a change in the frequency of the ringing voltageimpressed upon the conductors C112, C113 and C114 is effected. At itscontacts 124, 129 and 128, the relay R disconnects the F! interceptingpulse leads C151, C159 and C159 from the intercept pulsing conductorsC115, C119 and C111. At its contacts 123, .125 and 121, the relay R120connects the F2 intercepting leads C190, C19! and C192 to the interceptpulsing conductors C115, C119 and C111, respectively. The transfer relayR1!0, upon operating, complates, at its contacts H2, H4 and 9, obviouspaths for connecting the F-2 ringingleads C152, C154 and C159 to theringing current conductors C112, C119 and C114 respectively, these leadsbeing disconnected from the ring cut-01f for itself At its contacts 902,the relay R900 opens the circuit over which the Thus, the two transferrelays R'HO and R120 are caused to operate at the end of the conductorC11l at the contacts1l2', H4 and H8 when the relay R1l0 operates. At itscontacts 1| l', "3 and H, the relay R1l0 disconnects the F-l ringingleads C151, C153 and C155 from the ringing current conductors C112, C113and C114, respectively. Finally, at its contacts 1, H3 and "5, the relayR1I0 connects the F-l ringing leads Cl, C153 and C155 to the ringcut-off conductor 01.

The manner in which the coding relays 80 and the master timingrelays 85operate during the seventh to twelfth ringing periods, is exactly thesame as set forth above with reference to the operation of these relaysduring the first to sixth ringing periods, it being noted that duringthe seventh to twelfth ringing periods, ringing voltage of the frequencyF--2 is coded through operation of the coding relays 80 in the mannermore fully described hereinafter. At the end of the tenth ringingperiod, the relay R840 restores to complete an alternative holdingcircuit for the relay R140, this circuit extending from ground by way ofthe contacts "I and 1, C185, the contacts 843 and 823, C188, thecontacts 144, C184, the contacts 802-, C163, and the. winding of R140 tobattery. It will be noted that when this circuit is completed, bothwinding terminals of the relay R130 are connected to ground.Accordingly, the relay R130 is deenergized and restores. Upon restoring,the relay R130 opens, at its contacts 13i, a point in the initiallycompleted holding circuit for itself and a point in-the circuit forenergizing the relay R140. At its contact 132, the relay R130 opens afurther point in one branch of the holding circuit for the relay R800.At the end of the twelfth ringing period the relay R820 restores in themanner pointed out above, to open, at its contacts 823, the above-tracedalternative holding circuit for the relay R140. The relay R140. nowrestores to open, at its contacts 144, a further point in its holdingcircuit, and to recomplete, at its contacts 145, the path including theresistor 148 for short-circuiting its own winding. At its contacts 143,the relay R140 reprepares the operating circuit for the relay R130. Atits contacts 14 I, the relay R140 opens a further point in theabove-traced path for short-circuiting the winding of the relay R130. Atits contacts 142, the relay R140 reprepares the operating circuit forthe relay R800. Finally, at its contacts 141, the relay R140 interruptsthe circuit for energizing the relays R1l0 and R120 in series. The relayR110 in releasing, again connects the F--I ringing leads C15l, C153 andC155 to the ringing current conductorsC112, C113 and C114, disconnectsthe F-2 ringing leads C152, C154 and C158 from the enumerated ringingcurrent conductors, and connects the F-2 ringing'leads to the ringcut-oil conductor 01". The relay R120 in releasing, disconnects thecommon ringing current conductor C110 from the F-2' ringing converterlead C150, and reconnects this conductor to the F--l ringing converterlead C150. At its contacts 123 to 128, inclusive, the relay R120disconnects the F2 intercepting leads C180, 018i and C182 from theintercept pulsing conductors C115, C118 and C111, respectively, andreconnects these conductors to the F--l intercepting leads C151 to C158,inclusive. From this point on the manner in which the master timingrelays 85, the coding relays 80, the transfer control relays 85, andtransfer relays 80 operate during succeeding ringing cycles, is exactlythe same as described above.

The release of the interrupter 15 is under the control of the startrelay R100. More particularly, when the start conductor CH4 isdisconnected from ground, the 'relay R100 is'deenergized and restores todisconnect the release conductor C181 from ground. In response to thisoperation any operated ones of the relays R130, R140, R800, R8l0, R820,R830, R840, R850, R880, R800, R8 l0, R820, R840 and R850, aredeenergized and restored. If the relays R1l0 and R120 are operated atthe time the start relay R100 releases, these two transfer relays aredeenerglzed and restore in response to the release of the relay R140. Inthe event the relay R830 is operated at the time the start relay R100restores, this master timing relay is deenergized and restores inresponse to the release of the relay R820.

The ringing circuit control operations effected through operation of therelays embodied in the interrupter 15, in the manner explained above,will best be understood by reference to Fig. 10 of the drawings, whereinthe sequence of relay operations and the various circuit controlfunctions' effected thereby are plotted for one complete ringing cyclecomprising twelve separate and distinct ringing periods. It will beobserved from this chart that immediately the start relay R100 and thecoding relay R850 operate, a ground pulse is impressed upon the pick-uplead CH5 over a path which extends from ground by way of the contacts853, C118, the contacts 108 and the choke coil 135 to the pick-up leadCH5. As

' indicated by' the chart. the pick-up lead is disconnected from groundat the contacts 853 in response to the release of the relay R850,

:which, it will be recalled, occurs at the end of the first ringingperiod. From this chart it will also be observed that the pick-up leadCl 55 is again connected to groundjn response to the operation of therelay R850 which occurs during the seventh ringing period and isdisconnected from ground through the release of thisrelay at the end ofthis period.

Substantially concurrently with the application of ground potential tothe pick-up lead Cl l5, the F-l, code i intercepting lead C151 and thefrequency check lead C118, are connected to ground through operation ofthe relays R840,

R880 and R8l0, the path for grounding these leads extending by way ofthe contacts 8H and 823, the choke coil 8, the contacts'808, C880,and.the contacts 844'and 861, C115, and the contacts 124. As indicatedby the chart, this path is broken at the contacts 823 during the firstringing period in response to the operation of the timing relay R820.Ground potential is also impressed'upon the Fl, code 2 intercepting leadC158 for a portion of the fifth ringing period. Thus, when the relayR820 releases during this period, a path is established which extendsfrom ground by way of the contacts 833' and 823, the choke coil 8, thecontacts 808, C880, the contacts 845, 0118, and the contacts 128 to theintercepting'lead C158. This path is momentarily broken when the relayR830 releases during the fifth ringing period to cause the energizationand reoperation of the relay R8l0.

The relay R8i0 upon reoperating, however, completes an obviousalternative path for grounding the lead C158 at its contacts M3. Therelay R8i0 in operating also opens the path shortcircuiting the relayR820, causing the latter relay to reoperate and break at its contacts823, the alternative path over which the lead C158 is connected toground. Thus, a ground pulse is transmitted over the F-l, code 2intercepting lead C158 during the fifth ring ng Period. As indicated bythe chart, the F-I, code 3 intercepting lead C189 is similarly connectedto ground for a short time interval during the sixth ringing period.Thus, when the relays R9I 8 and R920 sequentially release during thisperiod, a path is established which extends from ground by way of theoperated contacts 833, the contacts 923, the choke coil 9", the contacts801, C819, the contacts 848, C111, and the contacts 128 to theintercepting lead C189. This path is broken at the contacts 923 inresponse to the reoperation of the relay R920 which occurs shortlyfollowing the release of the relay R930. As explained previously, thetransfer relay R120 is operated during the seventh to twelfth ringingperiods. Accordingly, ground potential is impressed upon the F--2intercepting leads C180, C18I and C182 forshortintervals during theseventh, eleventh and twelfth ringing periods, respectively, over pathswhich will be fully apparent in view of the foregoing explanation. Itwill be observed that the frequency check lead C180 is connected to theF--2, code I intercepting lead C180. Accordingly, ground potential isimpressed upon this check lead, concurrently with the application ofground potential to the F-2, code I intercepting lead C180, for a shorttime interval during the seventh ringing period.

It will be observed from Fig. 10 of the drawings, that a ground pulse isimpressed upon the pulse lead C889 during each cycle of operation of themaster timing relays 95. The path over which this pulsing lead isgrounded extends by way of the choke coil 888, C818, the contacts 922,

' of the second, ringing period in the manner preconductor C150, thecontacts 122, C110, the conand the parallel connected contacts 9I3 and933 g to ground. This path is obviously completedduring each intervalwhen therelay R920 is concurrently operated with either or both of thetwo relays R9I0 and R930. It will be understood that the frequency ofthe pulses impressed upon the pulsing lead C888 depends upon the speedwith which the master timing relays 95 sequentacts 8I3, C815, thcontacts 908, C818, the contacts 888 and 824, C113, and the contacts H3and 105 to the ringing le'ad C153. This circuit is interrupted at thecontacts 908 when the relay R900 operates at the end of the thirdringing period. The relay R900 upon operating at this time alsocompletes, at the contacts 805, a path for connecting th ringing leadC158 to the ring cut-oil conductor 01", this path extending by way ofthe contacts 905, C818, the contacts 888 and 824, C113 and the contactsH3 and 105, to the lead C153. At the beginning of the fifth ringingperiod, when the relay R900 again restores, the F--I, code 2 ringinglead C153 is again connected to the F--I ringing converter over theabove-traced circuit. Further, at the end of this ringing period, whenthe relay R900 reoperates, this circuit is broken at the contacts 808and the lead C153 is again connected to the ring cut-off conductor C11I'through the contacts 908. The circuit for connecting the F-I ringingconverter to the F-I, code 3 ringing lead C155 is completed when therelay R900 operates at the beginning-of the second ringing period, thiscircuit extending by way of the contially operate. In the preferredarrangement of 1 this interrupter, these timing relays are adjusted sothat the pulses impressed upon the lead C889 occur at a rate ofapproximately 80 impulses per minute.

It will also be observed from the chart illustrated in Fig. 10 of thedrawings, that the circuit connections extending between the ringingleads C15I to C158, inclusive, and the two converter leads C150 and C150are so arranged that the voltage of the F-I ringing converter :isimpressed upon the F-I, code I ringing lead C15I for the major portionof the first and second ringing intervals. The voltage of this samesource is also impressed upon the F--I, code 2 ringing lead C153throughout th third andflfth ringing periods. The F-I ringing currentsource is also connected to theF-I, code 3 ringing lead C155 during eachof the second, fourth and sixth ringing periods. More specifically, thecircuit for impressing the voltage of the ringing converter having theoutput frequency F-I upon the F-I, code I ringing lead C15l, iscompleted in response to operation of the relay R8l0, which occursshortly following the operation oi the start relay R100. This circuitextends from the high'- potential terminal of the F-I ringing converterby way of C150, the contacts 122, G; the conductor C, the contacts 122,G110, the contacts 903, the conductor C811, the contacts 828, C114, andthe contacts H5 and 101 to the ringing lead C155. This circuit is brokenat the contacts 903 when the relay R900 restores at the end .of thesecond ringing period in the manner pre- -viously explained. The relayR900, in restoring, also completes, at its contacts 904, an obvious pathfor connecting the ringing lead C to the ring cut-01f conductor C11I. Itwill be observed that the circuit for connectingthis ringing lead to theF-I ringing converter does not extend through any of the contacts of thecoding relays R830, R840, R850 and R860. Accordingly, the circuit iscompleted during each even-numbered ringing interval when the relaysR820 and R900 are operated. Also, this ringing lead is connected to thering cut-oil conductor C11I through the make contacts 828 of the relayR820 and the break contacts 904 of the relay R900 during eachodd-numbered ringing period when the relay R900 is restored.

As explained previously, at the end of the sixth ringing period all ofthe coding relays RBI 0, R820, R830, R840, R850 and R880 are released.With these relays in their respective restored positions it will beobserved that the three ringing current conductors C112, C113 and C114are all connected to the ring cut-off conductor C11I through the backcontacts of the coding relays R820 and R880. Thus, a ring cut-oifpotential is impressed upon all of these conductors during all silentperiods. From the foregoing explanation it will be recalled that thetransfer relays 'R1I0 and R120 are caused to operate at the end of thesixth ringing interval and remain operated until the end of the twelfthringing period. With these relays operated, the common ringing currentconductor C110 is connected through the operated contacts 12! to theconductor C150 which extends to the high potential terminal of the F-2ringing converter. Also, the F! ringing leads C15I, C153 and C155 areconnected through the make contacts of the relays R100 and R1l0 to thering cut-oil! conductor C11 I. C152, C154 and C156 are connected throughthe make contacts of the start relay R100 and the make contacts of thetransfer relay R1! to the ringing current conductors C112, C113 and C114respectively. It will be understood, therefore, that during the seventhto twelfth ringing D riods the voltage of the F2 ringing converter isimpressed upon the ringing leads C152, C154 and C156 during intervalswhich respectively correspond to the intervals when the voltage of theF! ringing current source was impressed upon the ringing leads C15!,C153 and C155, respectively, during the first to sixth ringing periods.Thus, from Fig. 10 of the drawings it will be seen that the voltage ofthe F2 ringing converter is impressed upon the F-2, code ringing leadC152 for the major portion of the seventh and eighth ringing periods.Similarly, the voltage of this converter is impressed upon the F2, cod 2ringing lead C154 for the duration of the ninth and eleventh ringingperiods.

Finally, the voltage of this converter is impressed upon the F-2, code 3ringing lead C156 for the duration of the eighth, tenth and twelfthringing periods. The circuits over which the voltage of this converteris impressed upon the" Operation of the system Considering now theoperation of the system as a whole, it may be assumed, for example, thata call intended for the substation B, associated with the line I I, isinitiated at the telephone station A. When the call is initiated, aconnection is extended to one of the selector switches of the system inresponse to operation of the line switch !5. More specifically, when theusual calling loop circuit is completed by way of the line !0, the lineswitch !5 operates to select a trunk line leading to an idle selectorswitch and to mark the calling line !0 as busy in the bank contacts ofthe connector switches having access thereto. Assuming, for example,that the selector switch 25 is selected by the line switch !5, when thetrunk line leading to this selector switch is seized, the calling loopcircuit is extended thereto and the selector switch is in condition torespond to the first 'series of impulses dialed at the callingsubstation. In response to the seizure of the selector switch 25,certain of the relays embodied therein operate in a well-known manner toimpress ground potential upon the tone start lead CH6. This start'leadis normally connected through the break contacts 68! of the relay R660to the the start conductor C692 extending to the No. 1 set of dial andbusy tone generators 690. As a result, operation of the apparatus 690 isinitiated. whereby the usual dial and busy tone sig- Further, the F--2ringing leads switch 25 to the established loop circuit extendingbetweenthe calling station A and the selector switch whereby the usual dialtone signal is reproduced by the receiver provided at the callingtelephone station.

When the first digit is dialed at the callingsubstation, the switch 25elevates its wipers to a position opposite the level of bank contactsterminating the trunks leading to the connector switches having accessto the desired line H. Following this operation and during theinterdigit pause between the first and second digits, the wipers of theswitch 25 are automatically stepped over the contacts of the selectedlevel until a trunk line leading to an idle connector switch of theselected group is found. Assuming, for example, that the connectorswitch 30 is the first available idle connector switch in the selectedgroup, when the wipers of the selector switch 25 are stepped intoengagement-with the contacts terminating the trunk line extending tothis connector switch, the subscribers loop circuit is extended to theswitch 30. The connector switch is thus conditioned to respond to thesecond series of impulses dialed at the calling substation. During theoperations just described, th dial tone lead C69! is disconnected fromthe loop circuit including the calling line !0 whereby the dial tonesignal transmitted to the substation A is terminated. When the connectorswitch 30 is seized in the manner just described, certain of the controlrelays embodied therein, and more particularly the line and hold relays,operate to complete a path for impressing ground potential upon theringing apparatus start conductor C! H, thereby to initiate operation ofthe ringing apparatus 185 and the ringing interrupter 15 in the eventthis apparatus is not already in operation. More specifically when thestart lead CH4 is grounded, a circuit is completed for energizing thestart relay embodied in the apparatus 185 and associated with the No. 1set of ringing converters. To this end the conductor C! !4 is includedin an obvious operating circuit for the relay R650. Upon operating, therelay R650 completes at its contacts 65! a circuit extending by way ofthe contacts 683 and the No. 1

startconductor C693 for energizing the start relay associated with theNo. 1 set of ringing converters embodied in the apparatus 185. Inoperating, the relay R650 also performs additional circuit controloperations referred to with particularity hereinafter.

In response to the dialing of the second digit at the calling telephonestation A, the wipers of the connector switch 30 are stepped verticallyto a position opposite the level of bank contacts at which areterminated the lines of the ten-line sub-group including the callingline H. Following this digit and during the inter-digit pause betweenthe second and third digits, the connector switch 30 is conditioned torotate its wipers step by step in accordance with the impulses of thecontrolled by certain of the relays in the selector When this thirddigit is dialed, the wipers of the switch 30 are operated intoengagement with the set of bank contacts terminating the called line H.During the interdigit pause between the thirdand fourth digits, circuitsare prepared for energizing the operating magnet of the frequency andcode selecting switch !20 in accordance with the impulses of the fourthand final digit. This operating magnet responds to the impulses of thefourth digit by operating the wiper !2! into engagement with the contactof the associated contact set, which has impressed thereon ringingvoltage of the particular frequency and code designating the calledsubstation B. If it be assumed, for example, that the ringer. providedat the called substation B is constructed to respond only to ringingcurrent of the frequency F-I and further, that the code designating thesubstation B is the code 2, th fourth digit dialedat the callingtelephone station A will necessarily comprise three impulses so that atthe conclusion thereof, the wiper I2 I will be left standing inengagement with th contact terminating the ringing leadC153. Thefrequency and code selecting switch I20 is provided with a releasemagnet, the operating circuit for which is prepared when the wiper I2Iis stepped off normal.

In accordance with conventional practice, suitable provisions are madein the connector switch 30 for testing the selected line II to determinethe idle or busy condition .thereof, for returning the usual busy tonesignal over the establishedv loop circuit 'to the calling subscriber inthe event the called line is busy, and for projecting ringing current ofthe selected frequency and code over the called line in the event thisline is idle at the time it is selected. In this regard it will beunderstood that the busy tone current utilized.

to signal the calling party in th event the called line is found to bebusy, is derived from the dial and busy tone generators 690 and isimpressed on the calling loop circuit over the busy tone lead circuit isprepared through operation of certain 0696. Assuming that the calledline is idle, a

of the relays embodied in the connector switch 30 for energizing thelower winding of the slowto-operate ringing control relay RI I0. Thiscir-' --cuit is thereafter completed at the beginning of the next cycleof operation of thecoding relays 90 embodied in the ringing interrupter15 when ground potential is impressed upon the pick-up lead CH5. More,specifically, when the relay R850 operates, following operation of thecon- I nector switch 30 to select the line II, the ringing relay RI I0is energized over a circuit which extends from ground by way of thecontacts 853, C118, the contacts 109, the choke coil 135, the pick-uplead CH5, certain of the contacts embodied in th relays of the connectorswitch 30, the private conductor I I0 of the selected line and throughthe winding of the cut-off relay conventionally embodied in the lineswitch 20 to battery. When this circuit is completed the cut-off relayembodied in the line switch.20 partially operates to disconnect thewindings of the line relay embodied in this line switch from theconductors of the called line I I, thereby to clear this line of theshunt impedance represented by the line relay windings. Completion ofthe circuit just traced also results in the energization and operationof the ringing control relay RI I0. This relay, upon operatingcompletes, at the X contacts I I2, a prepared holding circuit foritself, this holding circuit having previously been prepared throughoperation of certain of the other relays embodied in the connectorswitch 30. The relay RI I0 in operating, also completes a prepared pathfor impressing ground potential upon the private conductor I la of thecalled line, thereby to mark this line as busy in the bank contacts ofthe other connector switches having access thereto. At its contacts IIIand H3, the relay RI I0 completes a circuit for transmitting ringingcurrent of the selected frequency and code over the conductors of theline II. In th present assumed case, the circuit traversed by theringing current across the conductors of the line II, the con-- ductorIla, the contacts III and IN, the upper winding of the ring cut-offrelay RI00, and the exchange battery to the grounded terminal of theringing converter in use. It will be understood that in this case, sincethe ringing voltage is superimposed upon ground, the Y wiringillustrated in Fig. '1 is used in the ringing interrupter 15. Portionsof the ringing current transmitted over the line II are obviouslyshunted through the ringers respectively provided at the othersubstations associated with the line III All such ringers, which aretuned to respond to the frequency F- I, operate. Due to the codedcharacter of the ringing voltageimpressed upon the ringing lead-C153,however, the subscriber at the substation B is signaled that the call isintended for his substation, and the other subscribers served by theline and having ringers which respond to ringing current of thefrequency F I are informed that th call is not intended for theirsub-stations. A portion of the ringing current is also shunted in theusual manner over the calling loop circuit to energize the receiverprovided at the calling substation, thereby to indicate to the callingsubscriber that the called substation is being signaled. It willbe'apparent from the foregoing explanation that the code of the ringingcurrent projected over thelin II is such that swered at the calledsubstation B. More particularly, when the receiver at this substation isremoved from its supporting hook or cradle, a direct current conductivebridge is connected across the conductors Na and Nb of the line II tocomplete a circuit for energizing the upper Winding for the slow-actingring cut-oil relay RI00. This circuit may be traced as extending fromground by way of the ring cut-off conductor C11l, certain of the backcontacts of the coding relays or the relay R300, the conductor C113, thecontacts 1 I3 and 105, the ringing lead C153, the wiper I2I, thecontacts and H3, the conductor IIb, the. bridge across the conductorsofthe line II, the conductor Ila, the contacts III and IBI, and theupper winding of RI00 to battery. When its upper winding is energizedover this circuit, the relay RI00 operates to close its X contacts I03,thereby to complete a prepared holding circuit for itself. Thereafter,the relay RI00, at its contacts-WI and I05, opens two points in itsoperating circuit and two points in the above-traced circuit forprojecting ringing current over the line It. At its contacts I02 andI04, the relay RI00 completes the desired conversational circuit betweenthe calling substation A and the called substation B. When the relayRI00 operates it also functions to open the

